Two-dimensional variable length coding (2D-VLC) includes collecting or assuming the statistics of two dimensional block transform coefficient events that are each a run of the most likely-to-occur amplitude, e.g., 0, followed by another amplitude. The coding includes assigning variable length codes, e.g., optimal codes such as Huffman codes or Arithmetic codes, to each event. In the description herein, 0 is assumed to be the most likely-to-occur amplitude. The collecting of or assuming statistics includes tracking the quantized non-zero-valued coefficient amplitudes and the number of zero-valued coefficients preceding the non-zero amplitude, i.e., tracking the runlengths of zeros which precedes any non-zero amplitude along a specified path, e.g., a zigzag scan path for a block of coefficients, e.g., an 8 by 8 or a 16 by 16 coefficient block.
A variable length code such as an optimal code is then assigned to each of the events, with the most likely-to-occur element having the shortest number of bits, and the least occurring event coded using the longest number of bits. Table 1 below shows an example of a 2D-VLC table:
TABLE 12D-VLC codesRunlength of preceding 0's0123456..Coeff. Amp.1C10C11C12C13C14C15C16..2C20C21C22C23C24C25C26..3C30C31C32C33C34C35C36..4C40C41C42C43C44C45C46..5C50C51C52C53C54C55C56..6C60C61C62C63C64C65C66..7C70C71C72C73C74C75C76..8C80C81C82C83C84C85C86..9C90C91C92C93C94C95C96................................
where Cij is the codeword used to encode the event of the combination of j consecutive 0-valued coefficients followed by a single non-zero coefficient of amplitude i, j=0, 1, . . . and i=1, 2, . . . .
2D-VLC is used in common transform coding methods such as JPEG, MPEG1, MPEG2, ITU-T-261, etc., as follows. For motion video, an image is divided into blocks, e.g., 8 by 8 or 16 by 16 blocks. Each image is classified as interframe or intraframe. Interframe images are typically post motion compensation. The blocks of the image are transformed and the transform coefficients are quantized. The quantized transform coefficients are then coded along a specified path according to a 2D-VLC table. Typically, interframe and intraframe images have different 2D-VLC tables. The DC component is typically separately encoded. Furthermore, the 2D-VLC table may be truncated so that the least frequently occurring events use an escape code followed by a fixed length code. A special “EOB” code is used to indicate the end of a block when all remaining coefficients are zero.
Still images are similarly encoded, e.g., in the same manner as an intraframe image for motion video.
A table lookup may be used to implement a 2D-VLC scheme. Prior to the table look up, the runlength of zero amplitudes preceding any non-zero amplitude and the non-zero amplitude are determined. The table look up uses a 2D table for those likely events encoded using variable length encoding. An escape code together with a fixed length code is used for relatively less likely-to-occur combinations.
The advantage of 2D-VLC is that both the position of each non-zero-valued coefficient as indicated by the runlength, and the quantized amplitude value are coded simultaneously as a pair using one 2D-VLC table. This may result in shorter codes, i.e., codes that use fewer bits than using separate VLC tables for each non-zero-valued coefficient and for its amplitude.
Because of the widespread use of image coding, many patents have been issued on different forms of VLC. U.S. Pat. No. 4,698,672 issued Oct. 6, 1987 to Wen-hsiung Chen, one of the inventors of the present invention, for example described one form of a two-dimensional variable length coding method.
Extensions and variations to the common 2D-VLC method are known. For example, the ITU H.263 compression standard defines one such variation sometimes called three-dimensional VLC (3D-VLC). See PCT patent publication WO 9318616 published Sep. 16, 1993 titled PICTURE DATA ENCODING METHOD and also the ITU-T H.263 standard. In 3D-VLC, each symbol (“event”) is a triplet (LAST, RUN, LEVEL) that includes: LAST, a binary flag that indicates whether or not the current non-zero amplitude-value is the last non-zero coefficient in the block, RUN, the run-length of zero-value coefficients that precede the current non-zero amplitude, i.e., the number of zeroes since the last non-zero coefficient amplitude, and LEVEL, the current non-zero coefficient amplitude value. Thus, there is no need for a separate EOB codeword; whether or not the non-zero coefficient is the last one is incorporated into the event. A table lookup may be used to implement 3D-VLC.
One deficiency of 2D-VLC is that every non-zero-valued coefficient needs to be accompanied by a runlength code to identify its position, in the form of the number of preceding zero-valued coefficients.
In block based transform coding, the inventors have observed that there often is a region along the ordering in which non-zero-valued coefficients tend to cluster, i.e., there are often a number of consecutive non-zero-valued coefficients along the pre-determined path, especially at the beginning. This may especially occur in intraframe coding and high bit rate interframe coding. Each one of a number of such consecutive non-zero-valued coefficients would require the same number of codewords representing the position and amplitude. That is, 2D-VLC requires a separate runlength code, e.g., C10, C20, C30 . . . , etc., for each of the consecutive non-zero coefficients.
U.S. patent application Ser. No. 10/342,537 to inventors Chen et al., filed Jan. 15, 2003 and titled AN EXTENSION OF TWO-DIMENSIONAL VARIABLE LENGTH CODING FOR IMAGE COMPRESSION describes a method called the “Extended 2D-VLC Method” herein that includes encoding repetitions of some non-zero coefficient values. One variant of the Extended 2D-VLC method provides codes for all the possible amplitude variations of consecutive coefficients that follow a set of zero-valued coefficients. This effectively reduced the runlength to 1 for all cases. The difficulty of this approach is that there are enormous numbers of patterns that can be generated from the amplitudes of consecutive coefficients. For example, with 32 quantization levels as defined in many common video coding standards, there are in the order of 32n patterns that can be generated from n consecutive coefficients. As such, in a practical implementation, only a limited number of the most likely-to-occur non-zero amplitude values, such as 1 and 2, and a limited number of lengths of consecutive non-zero-values, such as 3 or 4 consecutive values, are regrouped for pattern matching.
The present invention provides coding methods that are appropriate where it is likely that there are clusters of non-zero-valued coefficients.
Many of the techniques described herein were first described in the related patent applications in connection with what we call “hybrid coding.” In coding, the inventors observed that not only are there are likely to be clusters of non-zero-valued coefficients, but that in some situations, such clusters are likely to occur in less than the whole region, e.g., in a low frequency region along the ordering of the coefficients, and that there is also likely to be a high frequency region where any non-zero-valued coefficients are likely to be scattered. With these observation in mind, the Basic Hybrid VLC Method of above-mentioned incorporated by reference U.S. patent application Ser. No. 10/869,229 to inventors Chen et al. was developed to encode the position and amplitude of quantized transform coefficients separately and takes advantage of the nature of the distribution of the transform coefficients in the low frequency and high frequency regions. Many of the other related patent applications dealt with either extensions of these methods, or with improved techniques for one or another region.
The Extended Hybrid VLC Method of incorporated by reference U.S. patent application Ser. No. 10/898,654 provides an alternative coding method for the high frequency region by taking advantage of the very few amplitude values in the high frequency region, especially, for example, for low bit rate and interframe applications.
In one embodiment of the above-mentioned Basic Hybrid VLC Method, two independent types of coding schemes are introduced to code the quantized coefficients along the path. A boundary is established along the path to define two regions, e.g., a low frequency region and a high frequency region. The boundary can be made adaptive to the video depending on a number of factors such as intraframe coding or interframe coding, standard definition television (SDTV) or high definition television (HDTV), complex scene or simple scene, high bit rate coding or low bit rate coding, and so forth. In one embodiment, the encoding of the quantized coefficients in the low-frequency region includes coding the positions of consecutive non-zero-valued coefficients and the positions of consecutive zero-valued coefficients using a run-length coding method of a first type and a run-length coding method of a second type. The encoding further includes coding the amplitude values and sign of the non-zero-valued coefficients. In the high-frequency region, in one embodiment, the encoding of coefficients in the high frequency region includes encoding the positions of either no consecutive zero-valued coefficients or runs of one or more consecutive zero-valued coefficients using a run-length coding method of a third type. The encoding further includes coding the amplitude values and sign of the non-zero-valued coefficients.
In one embodiment of the above-mentioned Extended Hybrid VLC Method, a coding method is used in the second region that takes into account that almost all non-zero-valued coefficients in the high frequency region are ±1. No amplitude coding is needed to encode runs of consecutive zeroes that end in a coefficient of amplitude 1. An exception (escape) code is included to encode those rare non-zero-valued coefficients that have values other than ±1.
In addition to hybrid coding, there also is a need in the art for coding when the whole region is the region where clusters are likely to occur, or where it is not appropriate to use two different coding methods.